The invention relates to amine-boranes. More particularly, the invention relates to a method of dehydrogenating amine-boranes. Even more particularly, the invention relates to a method of providing hydrogen for power generation sources, such as fuel cells.
Chemical hydrides for hydrogen storage are being explored as alternatives to high pressure tanks (gas or liquid), adsorbed hydrogen, and metal hydride fuels. Chemical hydrides have the potential to be packaged as non-pyrophoric, non-hazardous fuels for automotive applications. Hydrogen may then be generated from such hydrides under controlled conditions on-board and on demand. The spent fuel may then be regenerated either on-board or off-board.
Hydrogen storage materials should ideally have high hydrogen content and low molecular weight. Ammonia-borane (H3NBH3), having a molecular hydrogen storage capacity of 19.6 wt %, is therefore an attractive material for such applications. Because the molecule contains both hydridic and protic hydrogen atoms, solid ammonia-borane spontaneously loses H2 at temperatures above about 90° C. Ultimately, H3NBH3 can be dehydrogenated completely, forming ceramic BN, but temperatures in excess of 500° C. are required. Thermal decomposition of ammonia-borane in dilute solution (i.e., 0.15 M) at 85° C. initially affords the cyclic oligomers cyclotriborazane (B3N3H12) and borazine (B3N3H6). It has been demonstrated that preparation of B3N3H6 from H3NBH3 on a large scale can be achieved in high yield over 3 hours by simply heating a 1.1 M tetraglyme solution of ammonia-borane in the range from 140 to 160° C. while actively distilling the product molecules out of the reaction vessel. Isolated borazine can be thermally crosslinked at temperatures between 70° C. and 110° C. with concomitant H2 evolution.
It is possible to obtain a large amount of hydrogen from H3NBH3, but low energy (i.e., minimal heat input to initiate reactions) to utilize this fuel are only just being developed. For example, H2 has been liberated at room temperature from H3NBH3 and the related species dimethylamine-borane (HMe2NBH3) by addition of metal catalysts. For example, select Rh(I) species dehydrocouple HMe2NBH3 to form H2, along with the cyclic dimer [BH2NMe2]2 and acyclic aminoborane polymers.
The use of mesoporous scaffolds to template thermal H3NBH3 dehydrogenation has also been reported. No borazine was observed during this reaction, and the exclusive formation of linear polymers has been suggested. Such selectivity is important, since volatile products of dehydrogenation may negatively affect fuel cell catalysts used in series with a hydrogen storage material.
Currently, methods of dehydrogenating amine-boranes at low temperature require the use of expensive metal catalysts such as rhodium or ill-defined structure-catalysis relationships in the case of mesoporous scaffolds. Therefore, what is needed is a well-defined method of dehydrogenating amine-boranes at low temperatures without the use of metal catalysts.